The present invention relates to a method and a tool for treating at least the wall of a critical zone in a borehole, in particular a borehole for developing a hydrocarbon, gas, water, or analogous field.
A hydrocarbon, water, or gas field is generally developed using a drilling tool such as a drill bit which is rotatably driven from the surface, with transmission being via a drill pipe, or by a motor which is located at drilling tool level and which is mounted at the end of a drill pipe or a coiled tubing.
During the entire drilling operation, a drilling fluidxe2x80x94commonly known as xe2x80x9cmudxe2x80x9dxe2x80x94is pumped into the hole through the drilling tool. The mud cools the drilling tool and keeps the drilling debris in suspension to enable it to be evacuated to the surface. Another essential function of the mud is to ensure the safety of the well by providing hydrostatic pressure which is higher than the pore pressure of the formation, thus preventing any inadvertent upflow of gas or other fluids. However, the hydrostatic pressure must not exceed the fracture pressure of the rock.
Depending on the depth and type of formations encountered, that balance requires the use of muds of different densities which are incompatible with zones which have already been drilled at lesser depths. As a result, drilling has to be interrupted to position a casing to protect the zones which have already been drilled. Each interruption in drilling then corresponds to a reduction in hole diameter. If a number of critical zones are passed through, the well may have to be abandoned.
It would thus be desirable to have techniques available for at least temporarily treating such critical zones to limit the duration and cost of interruptions to drilling, and to do so with no substantial reduction in the hole diameter.
European patent EP-A-0,777,018 describes a technique for cementing a foundation shaft in the civil engineering industry.
In that document, a shaft is dug that is not necessarily of constant diameter because of the different hardnesses of the rocks through which the bit passes. In order to obtain a foundation shaft of substantially constant diameter, the wall of the shaft is cemented using a tool which is mounted above the bit. Thus, once the shaft is dug, the tool is activated to project a cement slurry against the wall of the shaft as the bit is raised, the body of the tool smoothing the slurry. In practice, the slurry must have a relatively fast setting time, and thus use is made of a Portland cement to which an activator, such as a silicate, has been added to increase the setting speed of the slurry which is pumped from the surface and guided by tubes which open laterally into the tool body.
Defining the composition of a cement slurry is a very complex problem which is difficult to master, in particular as regards selecting which additive(s) to add to the slurry to retard or activate setting, and in what proportion(s).
Such a problem can be solved without too much difficulty for cementing a foundation shaft which is only a few meters deep, as envisaged in the document cited above. The slurry which is pumped from the surface rapidly reaches the tool which projects it against the wall of the shaft.
In contrast, the problem becomes more difficult in the field of drilling to develop hydrocarbon, water, or gas wells which can be very deep, of the order of several hundreds of meters.
A critical zone which must be treated in a borehole may be located at any depth, and so a cement slurry must be controlled to set at the depth at which the critical zone is located, since the slurry must remain fluid until the critical zone is reached. Further, temperature is a parameter which influences slurry setting time, and must be taken into account since temperature increases with borehole depth.
In general, the invention consists both in a method which can control setting of a base fluid used to form a protective coating in a critical zone in a borehole whatever the depth at which the critical zone is located, and in a tool for carrying out this method.
The invention thus provides a method of treating at least the wall of a critical zone in a borehole, in particular a hole for developing a hydrocarbon, water, or analogous field, the method consisting in reinforcing the wall of the critical zone with a coating obtained from a base fluid which is pumped from the surface to a tool to be projected against the wall of the critical zone where it forms said coating once it has set, the method being characterized in that it consists in storing at least one additive or activator in liquid form in a reservoir of the tool, and in projecting the additive simultaneously with the base fluid against the wall of the critical zone to activate setting of the base fluid.
According to another feature, the method consists in raising the tool along the critical zone, while simultaneously projecting the base fluid and the additive by means of at least one injector, and in providing the tool with slip formwork located beneath the injector to keep the base fluid on the wall of the critical zone for a time equal to that required for the base fluid to set.
The invention also provides a tool for carrying out the method, the tool being mounted at the end of tubing to receive a base fluid which is pumped from the surface through the tubing and to project it against the wall of a critical zone detected in a borehole at any depth, the tool being characterized in that it comprises at least:
a reservoir in which an activator is stored to activate setting of the base fluid pumped from the surface and guided to the tool;
injectors to project both the base fluid and the activator simultaneously against the wall of the critical zone; and
a control means activated from the surface to control the operation of the injectors.
As an example, the tool is constituted by at least:
a connection module for connecting the tool to the tubing;
an injection module which comprises at least one reservoir containing an additive or activator in liquid form, and at least one injector to project both the base fluid and the activator simultaneously against the wall of the critical zone; and
a module forming slip formwork located beneath the injector to keep the projected base fluid on the wall in the critical zone for a time equal to that required for the base fluid to set, while the tool is being raised.
A number of embodiments of the tool can be envisaged. The tool can be used alone or in combination with a drilling tool.
When the wall of a critical zone is protected by a cement coating, the base fluid pumped from the surface is advantageously that described in the patent application filed on the same day by the Applicant, entitled xe2x80x9cControlling setting of a high-alumina cementxe2x80x9d (inventor: Michel MICHAUX).